In seismic exploration, the vibrations in the earth resulting from a source of seismic energy are sensed at discrete locations by sensors and the output of the sensors used to determine the nature of the underground formations. The source of seismic energy can be natural, such as earthquakes and other tectonic activity, subsidence, volcanic activity or the like, or man-made such as acoustic noise from surface or underground operations, or from deliberate operation of seismic sources at the surface or underground. Sensors fall into two main categories; hydrophones which sense the pressure field resulting from a seismic source, or geophones which sense vibration arising from a seismic source.
A prior art form of geophone is shown in FIG. 1. The geophone 10 consists of a moving coils 12, 13 mounted on a bobbin 14, a magnet 15, a pair of pole pieces 16, 18 with suspension springs 20, 22 and a housing 24 as shown in FIG. 1. The pole pieces 16, 18 and housing 24 are made of magnetically permeable material and form a magnetic field in which the moving coils 12, 13 are suspended.
When the earth moves due to the seismic energy propagating either directly from the source or via an underground reflector, the geophone, which can be located at the earth's surface or on the wall of a borehole which penetrates the earth, moves in the direction of propagation of the energy. If the axis of the geophone is aligned with the direction of motion, however, the moving coils mounted on the springs inside the geophone stay in the same position causing relative motion of the coils with respect to the housing. When the coils move in the magnetic field, a voltage is induced in the coils which can be output as a signal. The response of a geophone is frequency dependent and can be expressed as
                              e          g                =                                            Bl              ⁡                              (                                  ω                                      ω                    0                                                  )                                      2                                                              {                                  1                  -                                                            (                                              ω                                                  ω                          0                                                                    )                                        2                                                  }                            +                                                (                                      2                    ⁢                                                                                  ⁢                    ζ                    ⁢                                          ω                                              ω                        0                                                                              )                                2                                                                                      tan          ⁡                      (            φ            )                          =                              1            -                                          (                                  ω                                      ω                    0                                                  )                            2                                            2            ⁢                                                  ⁢            ζ            ⁢                          ω                              ω                0                                                                                      ω          0                =                              k            m                              Where    eg: induced voltage    B: magnetic flux density    l: length of the moving coil    ω: velocity of motion    ω0: natural frequency    k: spring constant    m: moving mass    ζ: damping factor
The internal damping is usually designed to be low and the total damping factor is adjusted by the use of a shunt resister externally attached and the factor is usually set to be about 70%.
One problem encountered with this design is how to increase sensitivity without dramatically increasing the size of the sensor, especially its diameter when considering use as a borehole sensor. Most prior art geophones use alnico magnets. To increase the sensitivity, a better magnetic material is needed. It is know that rare earth cobalt and/or neodimium iron boron (neogium) magnets produce larger magnetic flux than alnico; however, they have different characteristics and to obtain optimum flux density, the shapes of magnets need to be different for the different materials. A suitable shape for an alnico magnet is a relatively tall cylinder, whereas a rare earth cobalt magnet is preferably a relatively flat disc. To overcome the shape problem, a dynamic accelerometer was proposed as described in Japanese Patent Application No. 2-419184 and shown in FIG. 2. Flat, rare earth cobalt magnets 30, 32 are mounted face-to-face on a yoke 34 connected to the sensor housing 36 to achieve large flux density. A centre pole piece 38 is located in the space between the opposed magnets 30, 32 and a moving coil 40 is mounted on springs 42, 44 around the central pole piece 38. In this design, the natural frequency was chosen to be in the middle of the seismic frequency band and large damping is achieved by using imaginary short circuit connected across the coil output “40” of an operational amplifier 50 with appropriate resistors R1, R2 as shown in FIG. 3. While it is possible to attain a suitable size for such a sensor, the assembly cost has proved to be high.
Another problem with the prior art design of FIG. 1 is that the bobbin 14 should preferably be as light as possible. However, in the past this bobbin has been machined from metal which has proven both expensive and difficult to achieve the small thickness desired for size and mass limitations.